【作者】 徐晓光；

【导师】 姚强；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2009， 博士

【摘要】 生物质锅炉自出现以来,一直受到积灰问题的困扰。积灰造成锅炉效率低,可利用率低,严重阻碍了生物质燃烧技术的推广。本研究基于25 kW高温一维炉系统,利用表面温度可控的积灰采样装置,在不同的工况条件下,对燃烧过程中的积灰进行取样,并分析其特性。考虑到近年生物质锅炉的蒸汽参数逐步提高,本文首先将采样管壁面温度分别控制在550、600、650℃三个水平上,研究这一条件对积灰形成的影响。研究发现:采样管温度较低时,积灰的凝结成核机制得到增强;温度较高时,撞击熔融机制得到增强。结合生物质锅炉的发展现状,确定后续研究中采样管表面温度维持600℃。为研究生物质积灰的形成机理,再现成灰物质的沉积规律,依次延长采样时间,采集积灰样品并进行分析。分别研究草本、木质生物质的积灰形成过程,发现生物质的积灰速度呈现“快-慢-快-慢”的变化。通过微观上的分析,认为凝结成核机制、撞击熔融机制在不同积灰阶段的相互作用,是造成积灰速度变化的原因。通过化学成分的分析,发现高反应性的钾(K)是影响积灰形成的主要成灰物质。基于本文生物质积灰的研究结果,设计了通过实验研究烟气中的K浓度对积灰的影响。在火焰区喷入雾化的钾盐溶液,改变烟气中的K含量。实验结果表明:积灰倾向与烟气中的K浓度有良好的相关性。形貌和成分分析表明,K蒸汽通过两条途径促进积灰的形成:一是与硅铝酸盐结合,形成低熔点共熔物;二是沉积在换热器和飞灰颗粒的表面,增加二者的表面粘性。已有研究表明,高岭石对K具有良好的捕集效果。本文选用两种高岭石含量不同的煤种,分别与生物质混烧,研究煤中矿物成分对生物质燃烧过程中积灰形成的影响。结果表明,高岭石含量丰富的煤种与生物质混烧时,会捕集烟气中的K,生成低熔点共熔物。这一过程影响积灰的形成:抑制了凝结成核机制,但增强了撞击熔融机制。就本文的研究,麦秆与富含高岭石的煤种混烧时积灰趋势降低,而混烧低高岭石含量的煤种时积灰倾向增强。更多还原

【Abstract】 The feasibility of biomass boiler mainly depends on its impact on ash deposition. Deposition reduces the heat transfer efficiency, and is considered as an important problem challenging the development of biomass-combustion technology. In the present work, all experiments were performed in a 25 kW downflow one-dimensional combustor under well-set and reproducible conditions. A temperature-controlled deposition probe was self-developed and employed to collect deposit samples for further analyses like composition, morphology, size, etc.Firstly, the effect of probe surface temperature on ash deposition during biomass combustion was investigated, in which 550, 600 and 650℃were carefully selected here in order to meet the increasing steam temperature of biomass boiler in practice. It was found that the low surface temperature enhances the condensation and deposition of vapor-phase species as well as the thermophoresis-driven deposition of nucleated ultrafine particles, which is termed as condensation-nucleation mechanism. Meanwhile, the high surface temperature enhances the adhesion or sintering ability of the deposited particles because the occurrence of melting and is termed as impact-melting mechanism. The lowest ash deposition at 600℃of biomass fuels can be explained by the competing between these two mechanisms. In addition, the 600℃well meets steam parameter of current biomass boilers, and thus are used as probe temperature of main experimental runs.Then, the time-dependent deposition behavior of both woody and herbaceous biomass fuels was thoroughly studied by collecting ash deposit with various time-consumption. The deposition rate variation with respect to the time exhibits as follows: at an initial beginning the rate is quite high; then it becomes decreasing and later increases again to the initial level; finally, the rate increment slows down. For a point of micro-scale view, the competition between condensation-nucleation mechanism and impact-melting mechaniam was the reason for the recurrent deposition rates along the time. Based on the composition analysis of samples, the reactive potassium (K) was found as the key ash-forming element that subsequently affects the deposit formation.Thirdly, the effect of the potassium on ash deposition was extensively discussed by introducing the atomized solution of potassium acetate into the flame zone of one-dimensional combustor to control potassium concentration in flue gas. The results indicated that the deposit tendency is linearly dependent to the potassium concentration in flue gas. Morphological and chemical analyses of ash deposits further verified the role of potassium in deposition process. The significant increase of deposition tendency can be attributed to two routes: (1) the combination of aluminosilicate with the potassium that forms new eutectics with a lower melting temperature; (2) the condensation of potassium gas as well as the adhesion of potassium submicron particle on the surfaces of big particles or probe that even increases the work of adhesion between big particles.Finally, two kinds of Chinese coals, containing greatly different content of kaolinite, were used to co-firing with various biomass streams. The results indicated that, the high-kaolinite containing coal is effective to capture the potassium from flue gases. The change of ash deposition tendency during co-combustion can be attributed to the combination of the kaolinite from coal and the potassium from biomass, which formed the new eutectics with a lower melting temperature. In this work, it was found that the deposition tendency is decreased when co-firing straw and high kaolinite coal, but is increased when co-firing straw and low-kaolinite coal. It can be explained that the reaction of kaolinite and potassium innibites the condensation-nucleation mechanism, but enhances the impact-melting mechanism.更多还原